The present invention relates to a cooling cycle suited for use in automotive air-conditioning systems and a control method thereof More particularly, the present invention relates to a cooling cycle using a supercritical or transcritical refrigerant such as CO2 and a control method thereof.
The cooling cycle for automotive air conditioners uses a fluorocarbon refrigerant such as CFC12, HFC134a, or the like. When released into the atmosphere, fluorocarbons can destroy an ozone layer and cause environmental problems such as global warming. On this account, the cooling cycle has been proposed which uses CO2, ethylene, ethane, nitrogen oxide, or the like in place of fluorocarbons.
The cooling cycle using CO2 refrigerant is similar in operating principle to the cooling cycle using fluorocarbon refrigerant except for the following: since the critical temperature of CO2 is about 31xc2x0 C., which is remarkably lower than that of a fluorocarbon (e.g., 112xc2x0 C. for CFC12), the temperature of CO2 in a gas cooler or condenser becomes higher than the critical temperature thereof in the summer months where the outside-air temperature rises, for example, CO2 does not condense even at an outlet of the gas cooler.
The conditions of the outlet of the gas cooler are determined in accordance with the compressor discharge pressure and the CO2 temperature at the gas-cooler outlet. The CO2 temperature at the gas-cooler outlet is determined in accordance with the heat-radiation capacity of the gas cooler and the outside-air temperature. However, since the outside-air temperature cannot be controlled, the CO2 temperature at the gas-cooler outlet cannot be controlled practically. On the other hand, since the gas-cooler-outlet conditions can be controlled by regulating the compressor discharge pressure, i.e., the refrigerant pressure at the gas-cooler outlet, the refrigerant pressure at the gas-cooler outlet is increased to secure sufficient cooling capacity or enthalpy difference during the summer months where the outside-air temperature is higher.
Specifically, the cooling cycle using a fluorocarbon refrigerant has 0.2-1.6 MPa refrigerant pressure in the cycle, whereas the cooling cycle using CO2 refrigerant has 3.5-10.0 MPa refrigerant pressure in the cycle, which is remarkably higher than in the fluorocarbon cooling cycle.
An attempt has been made in the cooling cycle using supercritical refrigerant to enhance the ratio of the cooling capacity of an evaporator to the workload of a compressor, i.e., coefficient of performance (COP). U.S. Pat. No. 5,245,836 issued Sep. 21, 1993 to Lorentzen, et al. proposes an enhancement in COP by carrying out heat exchange between the refrigerant that has passed through the evaporator and the supercritical-area refrigerant that is present in a high-pressure line. In the cooling cycle including such an internal heat exchanger, refrigerant is further cooled by the heat exchanger to reach a throttling valve. This leads to still lower temperature of the refrigerant at an inlet of the throttling valve, which provides maximum COP.
In connection with the cooling cycle including an internal heat exchanger, JP-A 2000-213819 describes a method of controlling a throttling valve arranged upstream of an evaporator. This method allows control of the refrigerant temperature and pressure at the throttling-valve inlet to provide maximum COP.
However, such method of controlling the operating conditions of the compressor in accordance with the refrigerant temperature and pressure at the throttling-valve inlet raises the following inconvenience. Even when the outside-air temperature is constant, a variation in the air temperature in a cabin of a vehicle causes a variation in the heat receiving amount in the internal heat exchanger, which makes control providing maximum COP impossible.
Moreover, our study reveals that the conditions of providing maximum COP do not always correspond to those of providing maximum cooling capacity. An enhancement in COP is desirable in view of efficient operation of the cooling cycle. However, when it is desirable to give high priority to the cooling capacity, the operation of the cooling cycle under the maximum COP providing conditions cannot provide a target maximum cooling capacity.
It is, therefore, an object of the present invention to provide a cooling cycle for use in automotive air-conditioning systems, which can fulfill the most favorable performance in the operating environments. Another object of the present invention is to provide a control method of such cooling cycle.
The present invention provides generally a cooling cycle with a high-pressure side operating in a supercritical area of a refrigerant, comprising:
a compressor that compresses the refrigerant;
a gas cooler that cools the compressed refrigerant;
a throttling device that throttles flow of the cooled refrigerant;
an evaporator that cools intake air by a heat absorbing action of the cooled refrigerant;
an internal heat exchanger that carries out heat exchange between the cooled refrigerant and the refrigerant that passed through the evaporator;
a temperature sensor that senses a temperature of the cooled refrigerant between the gas cooler and the internal heat exchanger;
a pressure sensor that senses a pressure of the cooled refrigerant between the gas cooler and the internal heat exchanger; and
a controller that controls at least one of the compressor and the throttling device in accordance with the sensed temperature of the cooled refrigerant and the sensed pressure of the cooled refrigerant.
An aspect of the present invention is to provide a method of controlling a cooling cycle with a high-pressure side operating in a supercritical area of a refrigerant, the cooling cycle comprising:
a compressor that compresses the refrigerant;
a gas cooler that cools the compressed refrigerant;
a throttling device that throttles flow of the cooled refrigerant;
an evaporator that cools intake air by a heat absorbing action of the cooled refrigerant; and
an internal heat exchanger that carries out heat exchange between the cooled refrigerant and the refrigerant that passed through the evaporator,
the method comprising:
sensing a temperature of the cooled refrigerant between the gas cooler and the internal heat exchanger and a pressure of the cooled refrigerant between the gas cooler and the internal heat exchanger;
determining a control pattern of the cooling cycle in accordance with operating environments of the cooling cycle; and
controlling at least one of the compressor and the throttling device in accordance with the determined control pattern, the controlling step allowing adjustment of the temperature of the cooled refrigerant and the pressure of the cooled refrigerant.